Add PollPartialResultOptionFinder to extend PollOptionFinder

Introduce a dedicated finder for partial results, reusing the logic
of PollOptionFinder. This will be used in rake tasks to avoid code
duplication and make the intent clearer.

app/lib/poll_partial_result_option_finder.rb

@@ -0,0 +1,7 @@
+class PollPartialResultOptionFinder < PollOptionFinder
+  private
+
+    def existing_choices
+      question.partial_results.where(option_id: nil).distinct.pluck(:answer)
+    end
+end